A lot of us here are getting to be old hands, emphasis on getting old. I've
run into newer hands not as well versed in what was once called the "radio
art" as a lot of us are. This thread is like looking at over-reliance on "the
book" to the exclusion of "good engineering practice."
I've only been in the EMC line of work since 1983, but before that, I juggled
electrons as a hobby since 1958, did 15 years in military avionics, instructed
at the military Service School, etc etc. "The Book" tells us what we are to
do. Knowledge of what we are doing informs us how.
It is not so difficult to set right a perspective drawing little better than
Egyptian. And does anyone seriously think a drawing showing antennas lined up
in front of a reference ground plane means we have to TEST that way? Well,
yes, I ran into someone a number of years ago and talked him out of it. Not
long ago (in employer dimensions) a lab manager didn't let his techs use a
high pass filter between LISN's and EMC receivers. They bought several
receivers -- a few replacement input attenuators, too. Why didn't he? It
wasn't in the drawing.
These folks aren't stupid. But I wonder why if they are so afraid of making a
mistake they decline to learn the best ways -- better ones, anyhow -- and USE
them?
As Ken said... it's the WHY of it they need, not just "do things this way or
else". Not that this is new; how very many firms bought EXACTLY the equipment
in early issues because they didn't know how (or didn't want to be visible
doing it) to prove other things were as good?? Do I hear Bells?
Grrrr.
Anyone know ADS-37A? A really optimistic engineer once convinced himself (it
wasn't me!) to invert peak power and equivalent average for a Table III Part B
test. That shows a profound misunderstanding of basic principles, a kind
that is not uncommon enough. "Passing" get easier, though.
And WRT passing; in a lab not far away in Dilbert Space I spent a lot of time
telling an actually smart (but dumb) tech, "I don't want a test that passes! I
want a test that says where something FAILS."
Can I growl again? No?
Never mind; I've seen an RS103 antenna *practically* on fire. I won't say here
where or why. But I might get paid to explain it.
Rant over, nothing to see here folks, go about your business.
Cortland Richmond
KA5S
(Speaking for myself, not my employer and *certainly* not for Boskone)
On 8/30/2011 11:08 AM, Ken Javor wrote:
Precisely.
And Figure 2 hasn’t changed, in terms of the general layout of how
table-top equipment is laid out in a shield room, since well before
MIL-STD-462 (1967), going back to at least MIL-I-6181B, from 1953. Note that
MIL-I-6181B implies the existence of MIL-I-6181 and MIL-I-6181A, but I’m too
lazy to dig through them to ascertain if the set-up in those was similar or
not. And MIL-I-6181 was just one of several Service-unique specifications
covering EMI limits and test methods, and they were all quite similar. In
particular, all the testing of tabletop equipment was performed on ground
planes in shield rooms, and the ground plane was directly attached to the
shield room wall behind it.
As an aside, if you look at shielded enclosures of the
pre-MIL-STD-461-era,
they tend to be remarkably similar in size: close to twenty feet wide, only
about eight feet deep and high. The reason for that was that dipole antennas
were used from roughly 30 MHz to 1 GHz, and a 30 MHz dipole is 5 meters long,
and there was a required clearance between the dipole tips and the walls. But
the dipole only needed to be one foot removed from the test sample, not one
meter, as per MIL-STD-461. There was no technical need for absorber, and no
requirement. And of course the dipole could only be deployed horizontally, so
eight feet sufficed for depth and height.
So from 1953 – 1967 everyone understood how to do this test, and all the
figures in MIL-STD-462 (1967) simply carry this practice forward: the ground
plane is backed up to the shield room wall behind it. This is figures 1, 2,
CE01-1, CE02-1, RE04-2, and the other tests just showed details of the set-up,
with the ground plane up against the shield room wall either implied or
explicitly stated.
Because there was no requirement for absorber in 1967 and earlier,
absorber
would have been the exception, not the rule. So the major change with respect
to previous practice in MIL-STD-462D (1993) was the mandatory use of absorber,
and as Ed has pointed out, that is going to drive either a deeper ground
plane, or longer bond straps between the ground plane and wall or floor than
previously necessary, but it doesn’t change the fact that in the vast
majority of chambers, to make the room’s footprint as small as possible,
that means the ground plane backs up to the wall.
The point of all the above, and my previous posts on the same subject,
is
that what is being asserted to be an error or inconsistency is something which
has been accepted practice for well over a half-century.
Ken Javor
Phone: (256) 650-5261
-
----------------------------------------------------------------
This message is from the IEEE Product Safety Engineering Society emc-pstc
discussion list. To post a message to the list, send your e-mail to
<[email protected]>
All emc-pstc postings are archived and searchable on the web at
http://product-compliance.oc.ieee.org/
Graphics (in well-used formats), large files, etc. can be posted to that URL.
Website: http://www.ieee-pses.org/
Instructions: http://listserv.ieee.org/request/user-guide.html
List rules: http://www.ieee-pses.org/listrules.html
For help, send mail to the list administrators:
Scott Douglas <[email protected]>
Mike Cantwell <[email protected]>
For policy questions, send mail to:
Jim Bacher <[email protected]>
David Heald <[email protected]>
